The present invention relates to furnace ignition systems in general; and in particular, it relates to an ignition system for a fuel-burning appliance or furnace which eliminates the need of a conventional draft hood.
In addition to performing the functions of a conventional draft hood, the present invention also provides a stack damper when the appliance is off, controls the value of draft while the appliance is operating, establishes that there is no blockage of the flue before turning the fuel on, and provides loss-of-draft safety by shutting off the source of fuel if draft is lost and cannot be re-established within a predetermined time after the damper has been driven to the full open position. The loss-of-draft period is determined by an electronic circuit.
Draft hoods have been used with gas-fired appliances such as furnaces, water heaters, and the like for many years. A draft hood is a fitting which is normally located in the flue pipe leading from the appliance to the vent stack, or in the stack itself, which vents the combustion products from the fire box of the appliance to a chimney.
A draft hood serves a number of purposes in an installation of this type. In the case of a down draft or stoppage anywhere in the flue or stack beyond the draft hood, it permits the escape of combustion products into the ambient. A draft hood also neutralizes the effect of stack action of the chimney on the operation of the appliance by drawing air from the ambient and exhausting it through the chimney when the appliance is first fired.
Even though draft hoods have long been known to constitute a major heat loss to the outside, both when the furnace is being fired and when it is not, nevertheless, they have been widely used because the cost of fuel had previously been relatively low and because any alternative, particularly in residential usage, did not provide the degree of safety and reliability for the various conditions encountered in operation for a typical residential furnace.
Even with the advent of substantial increases in fuel cost, draft hoods have continued in use because of the difficulties in providing a system which accomplishes all of the purposes of a draft hood in a safe and reliable manner, and which does not require an expenditure on the part of the home owner for purchase and installation, which would not be offset by the savings in the cost of fuel.
The present invention provides an ignition and control system for a furnace which eliminates the need for a conventional draft hood while, at the same time, providing a stack damper when the furnace is off, controlling the value of draft when the furnace is fired, and shutting the furnace down in the event that draft is lost during any portion of the operating cycle. All of these functions and features are accomplished in a system which is economical to purchase and install, and which is safe for residential use.
Briefly, the present invention includes a damper plate which is movable under control between a closed and an open position. When a call-for-heat signal is generated, as by the closing of thermostat contacts in the room being heated by the furnace, the damper plate must be in the fully closed position or the system will not start. By returning the damper plate to the fully closed position at the end of each heating cycle, as will be more fully understood after reviewing this disclosure of the invention, all of the major mechanical and electronic components are proved and shown to be operative. Thus, a call-for-heat signal energizes a holding relay which, in turn, energizes a "run open" motor to move the damper off the fully closed position. At the same time, an initialization relay is energized to insure that the "run open" motor continues to be energized until the damper reaches the full open position, at which time the "run open" motor is de-energized, and a timer enable relay is energized.
The actuation of the timer enable relay energizes an electronic timer circuit for a predetermined time referred to as a "trial-for-draft". It is during this trial-for-draft period that an actual draft must be established as sensed by the draft controller, or the system will shut down. The timer circuit energizes an enable ignition relay during the trial-for-draft period, and it also enables the draft controller to be placed in operative circuit relation with the drive motors. This enable ignition relay energizes a fuel and ignition control circuit which permits fuel to flow through the main burner and also causes ignition.
Following the ignition of the fuel, a substantial draft will normally be created, causing the draft controller to energize a "run close" motor, thereby moving the damper plate off the full open position. Operation of the damper then continues under control of the draft controller so that the amount of draft is within design range. If at the end of the trial-for-draft period, the damper remains in the open position, indicating draft has not been established beyond a predetermined value, the system shuts off the fuel and locks out.
Once draft is established during the trial-for-draft period, the predetermined draft values define a desired draft range in which the system considers the amount of draft to be normal. If draft is less than a first value, a first switch within the Draft Controller energizes a damper motor to move the damper in the open direction to increase flow; and if draft is greater than a second value, a second switch within the Draft Controller energizes a damper motor to move the damper in the closing direction until a draft value is established within the design draft range. Either a single reversible motor can be used to actuate the damper, or, as in the case of the illustrated embodiment, separate motors can be used, each being actuated to rotate in a different direction to open and close the damper.
If neither of the draft controller switches is closed, the damper motor is de-energized, and this indicates that the draft value is within the design range. Hence, the damper remains at rest as long as draft is within the design range. If draft is lost, the system moves the damper to the full open position, and a loss-of-draft safety timing period commences. If draft is not re-established during this safety timing period, the system shuts off the fuel and locks itself out.
When the call for heat is satisfied, the system closes the damper and is prepared for another operating cycle, having tested the operativeness of all major components without failure. Any failure would have locked the system out. The various safety features of the system can only be understood in light of the particular circuitry of the system; and they will be described after the system is disclosed in detail. Of particular interest is the fact that in no instance will a single component failure permit a condition in which the damper plate is closed and the fuel and ignition circuit is energized. Also, during the trial-for-ignition, only an actual draft created by air flow can move the damper off the full open position and thereby avoid shut off. No draft during this period would indicate a blocked flue.
Other features and advantages of the present invention will be apparent from the following detailed description accompanied by the attached drawing.